A vinyl alcohol-based polymer (hereinafter, abbreviated as “PVA”) has been extensively used as a clear coating agent or a pigment coating agent for improving paper surface properties such as surface strength, smoothness, gloss, gas barrier performance, water resistance, printability, oil resistance.
A proportion of south-sea wood and used paper in a raw material for pulp has been increased, and along with this, paper surface strength tends to be reduced. Thus, in the light of performance of a coated paper, it is necessary to further improve paper surface properties. For a paper used for offset printing using water, high water resistance is required. Furthermore, for a barrier paper, high barrier performance is required. A representative example of a barrier paper is a base paper for a release paper. A base paper for a release paper is generally produced by coating the surface of a cellulose substrate with a PVA. Then, a release layer (silicone layer) is formed on the surface of the base paper for a release paper to afford a release paper. The PVA in the release paper serves as a barrier for preventing an expensive silicone from permeating a substrate. Nowadays, a PVA, which has not only barrier performance but also excellent adhesiveness between a paper substrate and a silicone layer, is required.
In order to improve productivity of a coated paper, a coating speed has increased, so that further reduction of viscosity of an aqueous solution of PVA is needed. For a blade coater, for example, a shear rate applied to a coating liquid during coating is frequently 106/s or more. At a high shear rate of 106/s or more applied to a conventional aqueous PVA-containing solution, the PVA in the aqueous solution tends to orientate and crystallize, leading to coating stripe and thus difficulty in uniform coating on a paper surface. Such a problem would be solved by, for example, reducing a polymerization degree of the PVA, reducing a saponification degree, or introducing an ionic group. However, a PVA with a low polymerization degree has the drawbacks of low coat strength and insufficient improvement in surface strength of a coated paper. A PVA with a low saponification degree or to which an ionic group has been introduced has the drawbacks of deteriorated water resistance of a dry coat, a low drying speed and so on.
Patent Reference No. 1 has described a base paper for a release paper coated with a silyl group-containing vinyl alcohol-based polymer which meets particular conditions. Coating with such a vinyl alcohol-based polymer is very effective for improving barrier performance of a cellulose substrate. However, it is not very effective for improving adhesiveness between a substrate and a silicone layer. Furthermore, viscosity of an aqueous solution tends to be equivalent to or higher than a common vinyl alcohol-based polymer, leading to problems in high-speed coatability.
Patent Reference No. 2 has described a cellulose substrate coated with a vinyl alcohol-based polymer in which a double bond is introduced in a side chain by acetalization. Coating with such a vinyl alcohol-based polymer is effective for improving adhesiveness between a substrate and a silicone layer, but viscosity of a solution tends to be equivalent to or higher than a common vinyl alcohol-based polymer, leading to problems in high-speed coatability.
Patent Reference No. 3 has described a coating agent for a paper made of a vinyl alcohol-based polymer prepared by polymerizing a vinyl ester at a temperature higher by 2 to 80° C. than a boiling point of a reaction solution and saponifying the vinyl ester-based polymer obtained. Such a vinyl alcohol-based polymer results in increase in the amount of 1,2-glycol bonds in a polymer main chain, slight inhibition of orientation and crystallization under a high shear force and excellent high-speed coatability. However, an aqueous solution thereof unfavorably has viscosity not so different from that of a common vinyl alcohol-based polymer, and water resistance lower than a common vinyl alcohol-based polymer.